function R = getsigma(mdl)
%GETREPR Gets the internal representation of sigma
%
% [ Syntax ]
%   - R = getsigma(mdl)
%
% [ Remarks ]
%   - The internal representation of the sigma for Gaussian model with
%     isotropic form covariance is a scalar giving the variance shared
%     by all dimensions. 
%
% [ History ]
%   - Created by Dahua Lin, on Dec 25, 2007
%

R = mdl.sigma;